CAMBRIAN-ORDOVICIAN EPISYENITES AND CARBONATITES IN SOUTHERN AND CENTRAL NEW MEXICO, USA: A POTENTIAL SOURCE OF REE

Rare earth elements (REE) (total REE, <2330 ppm) are found in brick-red, K-feldspar-rich episyenites and carbonatites throughout New Mexico and also contain anomalous concentrations of uranium (U, <9720 ppm), thorium (Th, <1380 ppm), niobium (Nb, <250 ppm) and heavy REE (<130 ppm Yb and <180 ppm Dy). The term episyenite is used to describe altered rocks that were desilicified (subsolidus dissolution of quartz) and metasomatized by alkali-rich fluids. Some episyenites contain elevated REE and other critical elements that could be a potential economic resource. Field observations and mapping indicate that episyenites are typically found as flat-lying pods or lenses (<300 m in diameter), pipe-like bodies (as much as 30 m thick), and dike-like bodies (<2 m wide, 400 m long). The contacts between the episyenite bodies and the host rocks vary from location to location, from very sharp to gradational. These episyenites could be part of a Cambrian-Ordovician magmatic event found throughout southern Colorado and New Mexico (McMillan and McLemore, 2001), characterized by the intrusion of carbonatites, syenites, monzonites, mafic dikes, associated K-metasomatism (i.e. fenites, episyenites) and Th-REE±U mineral deposits. There appears to be a multi-stage process for alteration. In the Caballo Mountains, the metasomatism is older than Cambrian as episyenite clasts are found in the Cambrian-Ordovician Bliss Formation that unconformably overlies episyenites and Proterozoic host rocks. However, ⁴⁰Ar/³⁹Ar dating of K-feldspars within some episyenites yields complex and intriguing age results that indicate age resetting or K-feldspar growth in later events. The Lemitar carbonatite yields a ⁴⁰Ar/³⁹Ar age of 517.7±0.7 Ma. Synchysite is a major host of light REEs in the episyenites (63 wt%), while heavy REEs are hosted in xenotime (16 wt%) and priorite (9 wt%). Textural evidence suggests that light LREE and heavy REE-bearing phases co-precipitated during metasomatism. Neodymiun isotopic data indicate that the Lemitar carbonatite was derived from a depleted (sublithospheric) mantle source (eNd at 520 Ma +4.8, T-DM 678 Ma), whereas the Caballo episyenites are much less radiogenic (eNd at 520 Ma ca. -5 to -8, T-DM ca. 1480-1780 Ma) and hence record a drastically difference source, dominated by Proterozoic lithosphere.